The present invention relates to a process for producing a high-purity erythritol crystal.
Erythritol (i.e., meso-erythritol) is useful not only as a sweetening agent but also as an intermediate product of medicines, industrial chemicals or the like. The erythritol can be industrially produced by culturing erythritol-producing microorganisms in an aqueous culture medium under aerobic conditions, for example, using glucose as a raw material.
The erythritol-containing culture solution contains various liquid or solid impurities. More specifically, the culture solution contains, as liquid impurities, by-products such as glycerol. Further, in the case where purified glucose obtained by enzyme-saccharization of starch, etc., is used as a raw material, the culture solution also contains, as liquid impurities, oligosaccharides such as di- or more saccharides inherently contained in the raw purified glucose, reaction products of these oligosaccharides, polysaccharides having a .beta.-1,4 bond which are composed mainly of glucose, or the like. In addition, the culture solution contains, as solid impurities, suspended fine substances in addition to microbe.
High-purity erythritol crystal can be produced by subjecting the above-mentioned culture solution to a process successively comprising a microbe-separating step, a chromatographic separation step and a crystallization step. As an example of these processes, there is known a method of separating and recovering erythritol from an erythritol-containing culture solution as described in Japanese Patent Publication (KOKOKU) No. 7-34748(1995).
Meanwhile, as described above, since erythritol is mainly used as a sweetening agent, it is preferred that the purity of erythritol produced be as high as possible. Further, from the standpoints of separating crystal from a slurry and preventing the agglomeration of fine powder produced due to inevitable fracture of crystal products, it is preferred that the crystal produced be in the form of a single-crystal having an appropriate size.
In addition, in the microbe-separating step of the above-mentioned process, when the removal of microbe and the suspended fine substances is insufficient, safe operations of the subsequent steps cannot be assured. For example, the unsatisfactory removal results in not only clogging of a resin column in the chromatographic separation step and failure of a heat exchanger by burning and sticking to the surface of its tubes or plates, but also lowering a purity of erythritol produced. Therefore, the microbe-separating step is extremely important in an industrial-scale process for the production of high-purity erythritol crystal. However, hitherto, the microbe-separating step in the process for producing erythritol has not been studied to a sufficient extent. For example, in Examples of Japanese Patent Publication (KOKOKU) No. 7-34748(1995), it has been only described to use a centrifugal separator in the microbe-separating step.
Further, in the industrial-scale production of high-purity erythritol crystal, it is also important that the production process thereof can be safely and stably conducted. As one step of the process to be stably conducted, there is exemplified a crystal separation step of separating an erythritol crystal from an erythritol crystal-containing slurry recovered from the crystallization step. That is, in the crystal separation step, there has been generally used a centrifugal separator. In this case, when erythritol crystal obtained by solid-liquid separation is unevenly distributed in the centrifugal separator, the stable operation of the centrifugal separator cannot be assured.
Furthermore, in the industrial production of high-purity erythritol crystal, it has also been demanded to produce such an erythritol crystal not only having a high purity but also free from inclusion of fin powder to avoid the agglomeration of crystal products.
The present invention has been attained in view of the above-mentioned circumstances, and is constituted by a group of inventions which are associated each other.